Electromagnetic locks, also referred to as maglocks, are well known locking devices that consist of an electromagnet and an armature plate. There are two main types of electric locking devices. Locking devices can be either “fail safe” or “fail secure”. A fail-secure locking device remains locked when power is lost. Fail-safe locking devices are unlocked when de-energized. Direct pull electromagnetic locks are inherently fail-safe. Typically, the electromagnet portion of the lock is attached to the door frame and a mating armature plate is attached to the door. The two components are in contact when the door is closed. When the electromagnet is energized, a current passing through the electromagnet creates a magnetic flux that causes the armature plate to attract to the electromagnet, creating a locking action. Because the mating area of the electromagnet and armature is relatively large, the force created by the magnetic flux is strong enough to keep the door locked even under stress. Typical single door electromagnetic locks are available with up to 1500 pounds dynamic holding force capabilities.
The magnetic lock relies upon the basic concepts of electromagnetism. Essentially, it consists of an electromagnet attracting a conductor with a force large enough to prevent the door from being opened. More specifically, the device makes use of the fact that a current through one or more loops of wire, i.e. a solenoid, produces a magnetic field. This works in free space, but if the solenoid is wrapped around a ferromagnetic core such as soft iron the effect of the field is greatly amplified. This is because the internal magnetic domains of the material align with each other to greatly enhance the magnetic flux density.
As mentioned, an electromagnetic lock operates under the premise of running an electric current though copper coils that surround a solid or laminate core of some ferrous material. This operation produces a magnetic field that permeates the core, and when the strike plate is introduced to the electromagnet, maximum magnetic holding force is created.
When the current through the coil is removed, the magnetic field collapses, but the core material maintains some amount of residual magnetism that continues to attract the strike plate. In the lock industry, this residual magnetism is not desired. Building code requirements often stipulate that the strike must be able to be separated from the electromagnet with minimal amount of force in a minimum amount of time. This can only be achieved with rapidly neutralizing the magnetic field through a degauss circuit. The process of degaussing removes or neutralizes the magnetic field of an object. Neutralizing a magnetic field almost always infers generating an opposing magnetic field. This is accomplished by reversing the direction of the current flowing through the coil windings.
Accordingly, there is a need for a degauss circuit that is capable of removing or neutralizing the magnetic field of an electromagnetic lock such that building code requirements are met whereby the strike can be separated from the electromagnet within the required time using the mandated amount of force.